Add host registration, heartbeat, and multi-host management

Implements the full host ↔ control plane connection flow:

- Host CRUD endpoints (POST/GET/DELETE /v1/hosts) with role-based access:
  regular hosts admin-only, BYOC hosts for admins and team owners
- One-time registration token flow: admin creates host → gets token (1hr TTL
  in Redis + Postgres audit trail) → host agent registers with specs → gets
  long-lived JWT (1yr)
- Host agent registration client with automatic spec detection (arch, CPU,
  memory, disk) and token persistence to disk
- Periodic heartbeat (30s) via POST /v1/hosts/{id}/heartbeat with X-Host-Token
  auth and host ID cross-check
- Token regeneration endpoint (POST /v1/hosts/{id}/token) for retry after
  failed registration
- Tag management (add/remove/list) with team-scoped access control
- Host JWT with typ:"host" claim, cross-use prevention in both VerifyJWT and
  VerifyHostJWT
- requireHostToken middleware for host agent authentication
- DB-level race protection: RegisterHost uses AND status='pending' with
  rows-affected check; Redis GetDel for atomic token consume
- Migration for future mTLS support (cert_fingerprint, mtls_enabled columns)
- Host agent flags: --register (one-time token), --address (required ip:port)
- serviceErrToHTTP extended with "forbidden" → 403 mapping
- OpenAPI spec, .env.example, and README updated

internal/auth/context.go

@@ -33,3 +33,31 @@ func MustFromContext(ctx context.Context) AuthContext {
 	}
 	return a
 }
+
+const hostCtxKey contextKey = 1
+
+// HostContext is stamped into request context by host token middleware.
+type HostContext struct {
+	HostID string
+}
+
+// WithHostContext returns a new context with the given HostContext.
+func WithHostContext(ctx context.Context, h HostContext) context.Context {
+	return context.WithValue(ctx, hostCtxKey, h)
+}
+
+// HostFromContext retrieves the HostContext. Returns zero value and false if absent.
+func HostFromContext(ctx context.Context) (HostContext, bool) {
+	h, ok := ctx.Value(hostCtxKey).(HostContext)
+	return h, ok
+}
+
+// MustHostFromContext retrieves the HostContext. Panics if absent — only call
+// inside handlers behind host token middleware.
+func MustHostFromContext(ctx context.Context) HostContext {
+	h, ok := HostFromContext(ctx)
+	if !ok {
+		panic("auth: MustHostFromContext called on unauthenticated request")
+	}
+	return h
+}